Until recently, soft contact lenses of the hydrogel type have been manufactured either by lathe cutting or spin casting. In the lathe cutting method, a lens blank or button of a substantially anhydrous hydrophilic polymer (xerogel) is mechanically cut and polished to a lens shape on a fine lathe, and thereafter is contacted with water or saline to hydrate the polymer and form the desired hydrogel lens. The mechanical steps utilized in the lathe cutting operation are similar to those used in the manufacture of hard contact lenses, except that allowance must be made for swelling of the lens during hydration of the polymer.
In the spin casting method, a small quantity of hydrophilic monomer mixture is placed in a concave, optically polished mold, and the mold is rotated while the monomers are polymerized to form a xerogel lens. The two optical surfaces of the lens are formed simultaneously during polymerization, the outer surface being formed by the concave mold surface and the inner surface being shaped by the joint actions of centrifugal force generated by the rotating mold and surface tension of the polymerization mixture. The lens produced thereby is contacted with water or saline to hydrate the polymer and form a hydrogel lens as in the case of the lathe cut lens.
More recently, an improved process for producing hydrogel contact lenses has been developed, which method is not only more economical than either the lathe cut method or the spin casting method, but it has the advantage of enabling a more precise control over the final shape of the hydrated lens. This new method comprises the direct molding of a monomer mixture wherein said mixture is dissolved in a non-aqueous, water-displaceable solvent, the mixture is placed in a mold having the shape of the final desired hydrogel (i.e., water-swollen) lens, and the monomer/solvent mixture is subjected to conditions whereby the monomer(s) polymerize, to thereby produce a polymer/solvent mixture in the shape of the final desired hydrogel lens. (The polymerization must be carried out in a non-aqueous medium because water inhibits the polymerization reaction ) After the polymerization is complete, the solvent is displaced with water to produce a hydrated lens whose final size and shape are quite similar to the size and shape of the original molded polymer/solvent article. Such direct molding of hydrogel contact lenses is disclosed in Larsen, U.S. Pat. No. 4,495,313 and in Larsen et al., U.S. Pat. No. 4,680,336.
In the Larsen patent, the water-displaceable diluents used are boric acid esters of polyhydric alcohols wherein the polyhydric alcohols have three or more hydroxyl groups. Alternatively, the polyhydric alcohols used may be a mixture of a polyhydric alcohol having three or more hydroxyl groups and a dihydric alcohol. See, for instance, the disclosure at Col. 3, lines 60 et seq. and Col. 4, lines 18-22.
The clear teaching of the Larsen patent is that the polyhydric alcohol used to prepare the borate esters for use in the direct molding process of hydrogel contact lenses must have three or more hydroxyl groups. While it is disclosed that dihydric alcohols can be used in admixture with tri- and higher polyols, the tri- and higher polyols are essential components.
This invention is based on the discovery that esters of boric acid and certain dihydric alcohols (as more fully defined below) can be used as water-displaceable diluents in a direct molding process for making shaped hydrogel articles such as soft contact lenses from polymer mixtures containing as the principal monomer one or more hydrophilic (meth)acrylates such as 2-hydroxyethyl methacrylate ("HEMA"). The invention provides processing advantages in the direct molding process for producing shaped hydrogel articles, including enhanced demoldability (i.e., the ability to open the mold after the polymerization with less force), which results in economic advantages such as a saving of labor costs, and a significant increase in yield because of a reduced proportion of surface defects in the molded articles that would cause rejection. It is believed that the enhanced demoldability and significant improvement in yield is related to the fact that the boric acid esters of diols that are employed in this invention have a lower surface tension than the preferred esters of the Larsen patent, No. 4,495,313, which reduces the adhesion of the polymer/solvent mixture to the mold.
An additional significant advantage that is imparted to the direct molding process by the water-displaceable esters provided by this invention is an enhanced ability to employ hydrophobic monomers (such as UV-absorbing monomers) in the polymerization mixture. When one tries to include hydrophobic monomers such as UV-absorbing monomers in a monomer/diluent mixture using as the diluent the preferred esters of the said Larsen patent, it is found that the hydrophobic monomers are often not soluble in the mixture.
Increasing medical awareness of the adverse affects of ultraviolet ("UV") radiation on the eyes has led to the introduction of spectacles, goggles, contact lenses, and intraocular lenses containing a means to absorb UV radiation. With respect to both contact lenses and intraocular lenses made from polymers (usually acrylic polymers), the preferred means for imparting UV absorbing capability is to make the lens from a copolymer that contains a copolymerized UV-absorbing monomer. Such monomers are disclosed, for example, in Beard et al., U.S. Pat. No. 4,528,311 and Dunks et al., U.S. Pat. No. 4,716,234. It would be desirable to impart UV-absorbing properties to contact lenses made by the direct molding process by including UV-absorbing monomers in the monomer/diluent mixture. This invention makes this desired end practical.